1. Field of the Invention
The present invention generally relates to composites and more particularly to thin copper foils supported on and bonded to improved carriers to form composites.
2. Description of Prior Art
Copper foil has been used for may years in the manufacture of printed circuits. The raw material for printed circuits usually is a laminate of copper foil and a thermosetting plastic, such as a phenolic or epoxy resin, which is often supported by paper, glass, cloth, or fibers. The supported plastic is usually applied in the uncured resinous state to the copper foil as a thick layer and is then heat and pressure cured to cause it to firmly bond to the foil. The finished laminate can have one or both exterior surfaces clad in this way with copper foil, whose thickness by weight, is usually about 1-3 oz. per sq. ft. The various methods of making printed circuits from such laminates are well known to those skilled in the art.
In recent years it has been found desirable to make laminates for printed circuits using copper foil that is much thinner than formerly. This is dictated by a new requirement that for certain applications, conducting elements be much smaller in width than previously and be spaced much closer together to facilitate circuit miniaturization. There is also a need to reduce the amount of copper removed by etching (to form the circuit conductor pattern) so as to reduce the cost of etchant and decrease etchant disposal problems.
Copper foils in thickness down to about 0.5 oz. per sq. ft. are made by electroplating the copper foil onto the surface of a slowly rotating steel drum, whose surface is specially prepared to prevent the copper layer from strongly adhering thereto. By controlling the speed of rotation of the drum and the electroplating conditions, the copper foil can be electroformed to a desired thickness. It is then peeled from the drum surface and wound onto a roll for further treatment, storage, or shipment. The surface of the foil formed away from the drum is considerably rougher than is the drum surface and is usually treated further to impart microscopic projections to it, so as to enhance its bondability to plastic during the subsequent lamination step.
When copper foil is mde in a thickness of about 0.5 oz. per sq. ft. or less, however, it becomes very difficult to handle. Thus, when it is cut into sheets and placed in the laminating press, it often wrinkles and tears unless a carrier is used to support it or unless extreme care is exercised. For ultra thin copper foils that are less than 0.5 oz. per sq. ft., the use of a thin carrier material such as a plastic film or a metal foil is often mandatory. Thus, in current practice, the desired ultra thin copper foil layer is electroformed on the surface of the foil or film carrier and may also be treated, while being supported there in order to enhance its subsequent bondability to the permanent plastic substrate in the laminating step.
Two general types of carriers are used for such purposes, those which are strongly bonded to the copper foil and those that are loosely adherent. Separation of the foil from a carrier is usually accomplished merely by peeling away the carrier, since the foil is usually only lightly attached thereto. But, when the copper foil and carrier are strongly bonded together, then their separation must usually be attained by drastic means such as by chemically dissolving the carrier away from the remaining thin copper foil.
After the copper foil is formed and treated on whatever carrier is employed, such foil and carrier are cut to the size necessary for the completed laminate. The carrier-supported foil is then assembled with the uncured, usually reinforced plastic (which is to constitute its backing) in a press, whereupon heat and pressure, for example, 200.degree.-400.degree. F. and 50-500 p.s.i. are applied to permanently bond the treated thin copper foil surface to the reinforced plastic base. Since the carrier is still attached to the other surface of the thin copper foil, it must then be removed either by peeling it away or by dissolving it off with chemicals, as described above.
In the case where the carrier is a plastic film, it must first be chemically prepared to receive a very thin metal layer by electroless deposition or by vacuum deposition and then the thin copper foil layer can be electroplated to the desired final thickness on the thin metal layer. The procedures required to chemically prepare the plastic for the initial metal layer, however, are intricate, time consuming, and require the use of expensive chemicals. Consequently, the use of plastic film as the carrier material is expensive and thin copper foil produced on such a carrier is quite expensive. The preparation of plastic film by vacuum deposition of metal thereon is also expensive and reliable methods for metallizing plastics without pinholes of various sizes have not yet been developed.
Because of these deficiencies, metal foil has been considered for use as the carrier. Thus, it is electrically conductive so that the metallizing step is averted. The particular foils that have been commercially used for this purpose have largely been copper. Before the thin copper foil layer can be deposited over the copper carrier, however, the surface of the copper carrier must be prepared to prevent too strong an adhesion from occurring. Various complicated procedures have been employed for this purpose, but have met with certain difficulties. One problem is that the copper carriers themselves are 2 mils or more thick and are made by electrodeposition, and thus are quite expensive. Like the plastic carriers, the high cost of thin copper foil produced on copper foil carriers has, therefore, severely restricted commercial applications.
One attempt to bypass these problems was made by Stroszynski in U.S. Pat. No. 3,660,190 dated May, 1972. This patent is directed to the production of a copper-plastic composite by roll laminating directly to the copper foil surface an adhesive and plastic layer, while the copper foil was still supported by the rotating drum or belt on which it was electroformed. However, the application of the adhesive or liquified plastic under roll pressure causes bleeding thereof through the pores invariably present in copper foil, and accordingly, the drum or belt surface would therefore become contaminated and require continuous renovation or replacement, thereby rendering the process economically unfeasible.
Accordingly, there is a need to provide an improved, inexpensive, simple and efficient method of making a very thin copper foil and plastic composite. The resulting composite should be especially quite useful in the manufacture of printed circuits and the like.